I've successfully set up Only Office on Docker. The email server is working fine, I can send and receive emails, no problems. Now I'd like to connect Outlook to my Only Office mailbox but I don't see any documentation on that. Is that not supported? I can't figure out what the user name should be...

This is a terrible message! Honestly, the mail server seemed for me like way to get out of my personal postfix-dovecot-spamassassin-dkim-maintenance hell. But if I can't connect from my mobile phone, this wonderful piece of software is quite useless for me.

Are there any plans to enable that (I am wondering where the problem is, as the mailserver uses standard tools - even dovecot - that would allow an imap connection from any client easily)!

we also need to implement this feature, because its really crucial for our customers.
You mention on GitHUB
ONLYOFFICE Mail Server is a full-featured mail server solution developed on the base of the iRedMail package,
And iRedMail supports IMAP and SMTP also for iRedAdmin-Pro. Can you please implement this feature soon?

Has there been any progress made on this topic? Without the ability to connect to third party clients, the mail feature of OnlyOffice subscriptions is effectively an unusable feature because messages cannot be received on mobile platforms or on platforms users are already accustomed to using. This isn't even a development issue so much as a systems administration one, you obviously have an SMTP and IMAP server already configured, it only need to be configured for end users.